Integrated circuits are formed on semiconductor wafers, which are then sawed into semiconductor chips. The semiconductor chips may be bonded onto package substrates. During the bonding process, the solder bumps between the semiconductor chips and the package substrates are reflowed. Conventional reflow methods include convection-type reflow and thermal compressive reflow. The convection-type reflow has relatively high throughput since a plurality of package substrates and the overlying dies may be bonded through the reflow at the same time. However, the convection-type reflow requires a long period of time to heat solder bumps. The resulting high thermal budget may cause significant warpage in the dies, and may possibly cause delamination between low-k dielectric layers.
The thermal compressive bonding requires a lower thermal budget than the convection-type reflow. However, the thermal compressive bonding has a very low throughput. During the thermal compressive bonding, a bond head picks up a die, flips the die, and attaches the die to a package substrate. The bond head then goes through a temperature ramp-up process to heat the die and the solder bumps that join the die and the package substrate. After the solder bumps are melted, the bond head goes through a cool-down process so that the solder bumps solidify. This process is repeated for each of the dies, and hence the throughput of the thermal compressive bonding is very low, which sometimes may be only 1/15 of the throughput of the convection-type reflow.